Method of providing aluminum with a flexible oxide coating



Patented Jan. 5, 1954 METHOD OF PROVIDING ALUMINUM WITH A FLEXIBLE OXIDE COATING Gerald W. Young, Schenectady, N. Y., and John L. Hain, Bethlehem, Pa., assignors to General Electric Company, a corporation of New York No Drawing. Application February 15, 1952, Serial No. 271,848

Claims.

The present invention relates to a method of providing aluminum with a flexible oxide coating. More particularly, it is concerned with a method of treating anodically oxide-coated aluminum surfaces to render the oxide coating more flexible.

It has been known for some time that aluminum surfaces can be anodically oxidized to produce thereon a coating which has good electrical insulating characteristics at elevated temperatures. However, when aluminum Wire is so treated, it is found that the anodic oxide coating is too brittle for use, for example, as insulation on magnet wire which is required to withstand a considerable amount of flexing without failure from an electrical standpoint. In fact, the ordinary oxide coatings obtained by electrolytic methods are too brittle to withstand even moderate flexing from an electrical insulation standpoint. Even in those cases where an organic enamel is applied over the aluminum oxide coating, the resultant composite coating is frequently no bet ter from the flexiblity standpoint than the straight aluminum oxide coating.

A primary object of the present invention is to provide a process for rendering more flexible the surfaces of anodized aluminum, that is, the oxide coatings obtained by anodically treating an aluminum surface. A further object of the invention is to provide a more flexible aluminum oxide coated magnet wire.

The present invention is based on the discovery that the above and further objects, which will become apparent from the following description of the invention, can be attained by subjecting the anodically formed oxide coating on an aluminum surface to an immersion in an aqueous solution of oxalic acid. More particularly, it has been found that when an anodized aluminum wire is immersed for a short time in a hot solution of oxalic acid the anodically formed oxide coating on the wire becomes more flexible, that is, less brittle and this improved flexibility characteristic is also possessed by the oxide coated wire after coating with any of the usual organic enamels such as the silicone resin enamels. Preferably the flexing treatment of the present invention is carried out by immersing the oxide-coated conductor in an aqueous oxalic acid solution containing from about 2.5 to oxalic acid for a time ranging from about 30 to 120 seconds. The oxalic acid bath is maintained at a temperature of at least 80 C. and preferably at a temperature of from about 84 to 95 C. Numerous experiments have shown that the desired increase in flexibility is obtained only by the use of an aqueous solution of oxalic acid at the more elevated temperatures. For example, neither hot water alone nor a butyl alcohol solution of oxalic acid will give the desired results.

Of the various anodically formed oxide coatings, it has been found that the flexing treatment of the present invention gives best results when employed on the coatings obtained by subjecting an aluminum or aluminum-surfaced wire to electrolytic treatment in an oxalic acid solution. In forming the anodic coatings, a dilute solution of oxalic acid containing for example from about 1 to 10% oxalic acid is used. Anodiza'tion of the aluminum is preferably carried out at a high current density ranging from about 159 to 450 amps. per sq. ft. and the wire is subjected to the anodizing treatment from about 1 to 3 minutes. The thickness of the anodic oxide coating depends of course upon the current density and time of treatment in the bath. In general, the oxalic bathshould be at a temperature of from room temperature up to about 50 C.

Ordinarily both the anodizing treatment and the flexing treatment can be carried out in one continuous operation. For example, the aluminum wire can be unwound from a feed spool and after being cleaned by passing through a bath composed of ethyl alcohol and ethylene dichloride to remove any traces of grease or die lubricant on the aluminum surface, the wire is run through wipers to remove the excess solvent from the wire, It is then dried and passed into the anodizing bath where an oxide coating is anodically formed due to the combined action of the electric current and the oxidizing agent, that is. the oxalic acid, As the wire leaves this bath, it is found that the oxide coating thereon is quite brittle. However, if the Wire is then passed immediately into an aqueous solution of oxalic acid of the above described composition, the oxide coating becomes somewhat thinner and considerably more flexible. The decrease in thickness of the coating is not, however, responsible for the increased flexibility since the coating of the same thickness obtained merely by anodic oxidation of the wire is much more brittle than that obtained by immersing a thicker coating in the flexing bath.

The following is a specific example of how the present invention can be carried into eiiect and the advantages thereof. A 32 mil diameter aluminum wire was anodized in a 5% aqueous solution of oxalic acid using a current density of 240 amperes D, C. per sq. ft., an immersion time of seconds and a bath temperature of 50 C. The

flexibility of the resultant wire was determined by winding it in the form of a helix on a tapered mandrel to obtain a range of elongation of the coating. The diameter of the helix on that portion of the mandrel at which cracks became visible in the oxide coating was noted. The elongation of the coating as a result of the winding was then calculated in percent using the formula where D is the overall turn diameter of the largest helix showing cracks in the oxide film, d is the overall diameter of the wire and E is the percent elongation. In general, a wire which has a coating which will not crack when elongated at least 25% is considered acceptable for magnet wire applications.

The anodically coated wire as obtained from the anodic treating bath had a flexibility equivalent to about 10% elongation of the coating. When this anodized wire was passed through a bath of pure water at 94 C., no improvement in the flexibility characteristics was noted. On the other hand, when the anodized wire was run through flexing baths respectively containing 2.5, 5, 7.5 and 10 percent oxalic acid at 94 C. a decrease in the thickness of the oxide film was noted although the decrease was substantially the same for all concentrations of the bath from 2.5% to 10% oxalic acid. The percent elongation of the coating without cracking steadily increased as the concentration of the baths increased from to oxalic acid. An even greater increase in flexibility was obtained as the concentration was approached. However, all of the wires treated in oxalic acid baths having a concentration of oxalic acid of at least 2.5% were found to be acceptable from the elongation or flexibility standpoint in that all of the coatings could be elongated at least 25% without cracking. In all of these tests, the time within the treating bath was maintained constant at 45 seconds. When a butyl alcohol solution of oxalic acid of concentration ranging from 2.5 to 7.5% was substituted for the aqueous solution, no appreciable change in thickness or flexibility of the aluminum oxide coating as compared with the anodic coating on the untreated wire was noted.

Oxide-coated aluminum wire having very good flexibility was obtained under the following conditions. A .002 diameter wire of aluminum moving at a speed of 4-. feet per minute was passed through a 5 foot anodizing bath composed of a 5% aqueous solution of oxalic acid at 50 C. and then through a 3 foot flexing bath composed of a 5% solution of oxalic acid in water and thereafter washed and dried. Currents of from 10 to 14 amperes were employed in the anodizing bath. The flexing bath was maintained at a temperature of 84 C. The resultant wire was coated. with an exceedingly flexible oxide coating which showed no signs ofcracking under a 'Z-pcwer microscope when flexed to an elongation of 50%. The thickness of the oxide film ranged from about .06 to .11 mil. Some of the product was coated with the resin, specifically a polymeric monochlortrifluorethylene resin, and subjected to the same flexibility test. There was no sign of cracking of the coated wire under a 36-power microscope even after 50% elongation.

While the invention has been particularly described with reference to the treatment of an aluminum wire, it is to be understood that it is not limited thereto. The invention is applicable to any aluminum-surfaced conductor and has been successfully applied, for example, to conductors composed of a copper core and an aluminum or aluminum alloy surface coating. The resultant products can be employed in the manufacture of electrical coils and the like. If desired, additional insulation of an organic nature may be applied, one such product being described in Patent 2,542,069Young.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of providing an aluminum surface with a flexible oxide coating which comprises anodically oxidizing the surface in a dilute aqueous solution of oxalic acid and thereafter immersing the oxide-coated surface in a flexing bath consisting of an aqueous solution of oxalic acid at a temperature of from about C. to C.

2. The method of providing an aluminum-surfaced conductor with a flexible insulating oxide coating which comprises anodizing the surface of said conductor in an aqueous solution of oxalic acid at a temperature not exceeding 50 C. and thereafter immersing the anodized product in a flexing bath consisting of an aqueous solution of oxalic acid containing from 2.5 to 10% by weight oxalic acid, said solution being held at a temperature of from about 80 to 95 C.

3. The method of providing an aluminum-surfaced conductor with a flexible oxide insulating coating which comprises anodizing said conductor in a 5% oxalic acid solution in water at a current density of about 240 amps. per sq. ft. for about 75 seconds and thereafter passing the anodized conductor through a flexing bath consisting of an aqueous solution of oxalic acid containing from about 2.5 to 10% oxalic acid, said solution being held at a temperature of from about 84 to 94 C., said anodized conductor being immersed in said flexing bath for about /2 to 2 minutes.

4. In a continuous method of providing an aluminum-surfaced conductor with a flexible oxide coating, the steps which comprise anodizing an aluminum-surfaced conductor in an anodizing solution consisting of an aqueous solution of oxalic acid and immediately thereafter passing the anodized conductor through a flexing bath consisting of an aqueous solution of oxalic acid at a temperature of from about 80 to 95 C.

5. In a continuous method of providing an aluminum-surfaced conductor with a flexible oxide insulating coating, the steps which comprise anodizing an aluminum-surfaced conductor in a 5% oxalic-acid solution in water at a current density of about 240 amps. per sq. ft. for about '75 seconds and thereafter passing the anodized conductor through a flexing bath consisting of an aqueous solution of oxalic acid containing from about 2.5 to 10% oxalic acid, said solution being held at a temperature of from about 84 to 94 C., said anodized conductor being immersed in said flexing bath for about /2 to 2 minutes.

GERALD W. YOUNG. JOHN L. HAIN.

Name Date Blackmun Sept. 17, 1946 Number 

1. THE METHOD OF PROVIDING AN ALUMINUM SURFACE WITH A FLEXIBLE OXIDE COATING WHICH COMPRISES ANODICALLY OXIDIZING THE SURFACE IN A DILUTE AQUEOUS SOLUTION OF OXALIC ACID AND THERAFTER IMMERSING THE OXIDE-COATED SURFACE IN A FLEXING BATH CONSISTING OF AN AQUEOUS SOLUTION OF OXALIC ACID AT A TEMPERATURE OF FROM ABOUT 80* C. TO 95* C. 